At least 18 people arrested in the US city of St Paul following acquittal of officer who shot and killed a black man.

At least 18 people have been arrested in the US city of St Paul during a mass protest that erupted following the acquittal of a police officer who shot and killed an African American man.

Protesters were rallying outside the Minnesota State Capitol on Friday night and into Saturday, after a jury acquitted police officer Jeronimo Yanez of manslaughter and other charges, in the fatal shooting last summer of Philando Castile during a traffic stop.

The protesters eventually left the capitol and began marching through the streets, with hundreds splintering off and blocking a highway for about two hours.

The Minnesota state police said officers began arresting the protesters past midnight, after issuing three warnings for them to get off of a major highway in the city.

The protesters were booked on charges including being a pedestrian on the busy freeway.

Castile's death was captured via a Facebook livestream launched by his girlfriend seconds after he was shot by the police officer.

In it, Castile was seen bleeding to death in the driver's seat.

His death sparked outrage against police and allegations that it was racially motivated.

But no video existed of exactly what happened inside the car.

That left a jury to decide whether they believed police officer Yanez on the witness stand, and they opted on Friday to acquit him.

In the US, legal cases are usually decided by a panel of 12 people sworn to give a verdict based on evidence submitted to a court. Their decision is administered by a judge.

Yanez testified that Castile, a school cafeteria worker, was reaching for his gun despite his commands not to pull it out. Yanez said he feared for his life.

Yanez had initially singled out Castile for a traffic stop because the officer thought he bared a resemblance to a robbery suspect.

'Tragic, needless death'

Castile, however, had no criminal record.

Castile volunteered that he was legally carrying a gun. "Sir, I have to tell you that I do have a firearm on me," he said.

Castile's girlfriend, Diamond Reynolds, and her then four-year-old daughter were also in the car when the officer shot Castile.

The officer asked Castile not to pull out the handgun. But moments later Yanez fired seven shots while Castile was still buckled into his seat. Reynolds, the girlfriend, said Castile had been trying to pull out his wallet.

Yanez said he feared for his safety and thought Castile was reaching for the gun.

But Ramsey County Attorney John Choi, who filed charges against the officer, declared such fear unreasonable.

Valerie Castile said her son was murdered.

"I'm mad as hell right now. Yes, I am," Castile's mother tearfully told a group of reporters after the verdict was issued. "The system continues to fail black people."

Glenda Hatchett, a lawyer who represented Valerie Castile, said her son had suffered a "tragic, tragic needless death".

"This time we had a young man who had no criminal record," she said. "This time there should have been, in our opinion, a very, very different outcome."

Community activists expressed anger and disappointment.

"It was a clear-cut case," Jaylani Hussein, chief of the Minnesota chapter of the Council on American-Islamic Relations, said on local TV station KSTP.

"It sends a very harsh message that we still have major race issues in this country."